1. Field of the Invention
The invention relates to a road construction machine, in particular a road milling machine, a crawler track unit for a road construction machine, as well as a method for tensioning a chain of a crawler track unit.
2. Description of the Prior Art
Tracked vehicles are provided with a chassis with several crawler track units, said chassis carrying the machine frame.
A crawler track unit is comprised of the following elements:                a chain with an upper strand and a lower strand made of metal, or with additional track pads for road operation,        a drive wheel, which transfers the torque from a drive engine to the chain,        a guide wheel, which is preferably provided with an adjustment device enabling the reciprocal spacing of the axles of the drive wheel and guide wheel to be adjusted for adaptation to a current length of the chain,        track rollers, which carry the machine frame and roll on the chain,        possibly additional support rollers, which support the return movement of the upper strand of the chain,where, in addition to the adjustment device, a spring element may be provided which allows, using the adjusted spacing between the axles of the drive wheel and guide wheel as a basis, the spacing to be shortened against the spring force in the event of an operation-related increase of the chain tension to prevent any tension peaks in the chain.        
In a crawler track unit, the drive wheel is normally located in the rear position when seen in the direction of travel, so that the lower strand of the chain is subject to tensile stress. In this arrangement, returning of the chain under load causes increased wear of the chain.
To adjust the required chain tension, which enables low chain wear on the one hand and prevents the chain from disengaging on the other, it has to date been known to use a chain tensioning device consisting of a grease tensioner in combination with a spring element. The spring element enables the spacing between the axles of the drive wheel and guide wheel to be shortened in the event of an operation-related increase of the chain tension to prevent any tension peaks in the chain. The grease tensioner uses essentially incompressible grease and serves to adjust the reciprocal spacing of the axles of the drive wheel and guide wheel for adaptation to a current length of the chain. This means that, when the chain has lengthened, a compensation for the change in length is effected via the grease tensioner, where the slack of the chain is determined first and then grease is fed manually via the grease tensioner until the chain shows a desired slack.
It is of disadvantage in this design that these adjustment procedures are performed manually and need to be carried out separately for each crawler track unit of a tracked vehicle, which results in a significant loss of time.
It is of disadvantage in this design also that these manual maintenance procedures are frequently performed too infrequently or even too frequently.
If the maintenance procedures are performed too infrequently, there is the risk that the chain may disengage or may be subject to excessive wear. If the maintenance procedures are performed too frequently, the resulting loss of time is of disadvantage. Apart from that, performing the maintenance procedures manually also involves the risk of mistakes frequently being made when measuring the chain slack.
Another possibility is to control the chain tension, for instance, purely hydraulically, where an additional spring element is not normally provided in such systems. A piston-cylinder unit keeps the chain permanently tensioned. When the crawler track unit drives over an obstacle (as an example of an operation-related increase of the chain tension), a pressure-limiting valve connected to the cylinder of the piston-cylinder unit can release pressure, in which case the piston retracts.
It is of disadvantage in this design that the permanent, relatively high chain tension results in an increase of chain wear. The high degree of wear is caused by the requirement of an increased feed pressure being permanently available which causes the piston to extend again after deflection on account of an operation-related increase of the chain tension. A problem also exists during reverse travel, as the chain may easily disengage during deflection of the guide wheel. In reverse travel, the chain tension extends across the upper strand of the chain and across the guide wheel, lessening only on the lower side in the load strand. If pressure is then released by the pressure-limiting valve as a result of an overload situation, there is the risk of the chain disengaging.
A chain tensioning system for a chain of a crawler track unit is described in DE 102 57 405 A1 and U.S. 2003/0117017, by means of which a relatively high chain tension is maintained in an excavator during operation. This arrangement prevents the excavator from rolling backwards or rolling forwards on the chain when in operation.
If the excavator is merely moved without being in operation, however, the chain tension is reduced in order to reduce the wear of the crawler track unit.